1. Field of the Invention
The present invention relates generally to methods of correcting defects in phase shift mask patterns, and more specifically, to a method of correcting defects in a phase shift mask patten having bump defects (excess shifters) and divot defects (missing shifters).
2. Description of the Background Art
Conventionally, as a general method of correcting defects in a mask pattern, there is known a method utilizing a laser beam or a focused ion beam (FIB). FIGS. 36-38 are cross sectional views for use in illustration of a conventional general method of correcting black bump defects in a metal light shielding film of a mask. Referring to FIGS. 36-38, the conventional method of correcting black bump defects in the metal light shielding film will be described.
As illustrated in FIG. 36, a usual mask has a metal light shielding film 43 of a prescribed pattern formed on a mask substrate 41. A black bump defect (excess metal film) 42 is generated when the metal light shielding film 43 is formed. FIGS. 39-41 are views showing a manufacturing process for use in illustration of one example of causes for generation of the black bump defect 42. In order to form the metal light shielding film 43 having a prescribed pattern (see FIG. 36), a metal light shielding film layer 43a is formed on the entire surface of the mask substrate 41 as shown in FIG. 39. After resist 44 is formed on the metal light shielding film layer 43a, a prescribed portion of the resist 44 is exposed. At the time of exposure, if an alien substance 45 is present on the resist 44 at the position to be exposed (see FIG. 39), the portion of resist 44 positioned under the alien substance 45 is not exposed. If the region to be originally exposed is thus not exposed, unnecessary resist 44a as shown in FIG. 40 is formed when the resist 44 is developed. When such necessary resist 44a is formed, and the metal light shielding film layer 43a is subjected to a dry etching utilizing as mask the resist 44 and 44a, the metal light shielding film black bump defect 42 as shown in FIG. 41 is formed. Thus, conventionally, when the metal light shielding film 43 is formed, the metal light shielding film black bump defect 42 is generated.
A metal such as Cr or a metal compound such as MoSi is used as a material for the metal light shielding film 43. In order to correct the black bump defect 42 in the metal light shielding film, as shown in FIG. 37, an Nd:YAG laser beam 9 is irradiated upon the black bump defect 42 in the metal light shielding film. Thus, the black bump defect 42 is evaporated as shown in FIG. 38 and removed away. The Nd:YAG laser beam used has a wavelength of 532 nm and irradiated for about 200 mJ-300 mJ.
FIGS. 42-44 are cross sectional views of structures for use in illustration of a general method of correcting a white divot defect (missing shifter) in a metal light shielding film of a mask according to a conventional technique. Referring to FIGS. 42-44, this conventional method of correcting a white defect in the metal light shielding film will be described.
As illustrated in FIG. 42, a metal light shielding film 53 of a prescribed pattern is formed on a mask substrate 51. A white divot defect 52 is generated when the metal light shielding film 53 is formed. FIG. 45 is a cross sectional view for use in illustration of one example of causes for generation of such a conventional metal light shielding film white divot defect 52. Referring to FIG. 45, in order to form the metal light shielding film 53 (see FIG. 42), a metal light shielding film layer 53a is formed on the entire surface of the mask substrate 51 by means of sputtering or the like. When the metal light shielding film layer 53a is formed, the pinhole (white defect) 52 is generated. If the region in which the pinhole 52 is located is a region to be formed as the metal light shielding film 53 (see FIG. 42), the metal light shielding film white divot defect 52 is generated in the metal light shielding film 53 eventually formed. Thus, in a conventional process of forming the metal light shielding film layer 53a, the metal light shielding film white divot defect 52 is generated.
In order to correct the white divot defect 52 in the metal light shielding film, a deposition gas 6 formed of a hydrocarbon-based gas is introduced into the region of the defect as shown in FIG. 43. An FIB 5 is irradiated upon the defect region while scanning the region. A carbon film 52 is thus formed to fill the white divot defect 52 in the metal light shielding film as shown in FIG. 44. Ga ions of about 20-30 keV are utilized as the FIB 5. A metal carbonyl-based gas or an organic metal-based gas may be used other than the hydrocarbon gas as the deposition gas 6.
The above-stated method has been known as a method of correcting defects such as the black bump defect 42 and the white divot defect 52 produced in a conventional general mask. When the above-stated method of correction is utilized for defects in a phase shift mask without modification, however, the following problem is encountered.
Now, a phase shifter process utilizing a phase shift mask will be described. According to the phase shifter process, a phase difference is provided to light passing through a photomask, in order to further improve the resolution of a lithography technique. Thus, the light intensity profile can be improved. FIGS. 46 and 47 are schematic views for use in illustration of the principles of the phase shifter process. Referring to FIG. 46(a), according to a conventional usual photomask, a metal light shielding film 43 of chromium constituting a light shielding portion is formed on a transparent mask substrate 41 such as a silica substrate. More specifically, the region in which the metal light shielding film 43 is formed constitutes the light shielding portion, while the region without the metal light shielding film 43 constitutes light transmitting portions 100a and 100b. In this manner, conventionally the metal light shielding films 43 are formed a prescribed distant apart from each other, and a repetitive pattern of lines and spaces is formed. The amplitude of light transmitted through thus formed mask pattern ideally takes a waveform as shown in FIG. 46(b) at A1. More specifically, ideally, light transmitted through the metal light shielding film 43 is 0 and the entire light passes through the light transmitting portions 100a and 100b. In such an ideal state, the amplitude of transmitting light takes the waveform A1 as shown in FIG. 46(b). However, the amplitude of actual transmitting light takes a waveform A2a or A2b as shown in FIG. 46(c) due to the diffraction of the light. The waveform A2a is the amplitude waveform of the light transmitted through one light transmitting portion 100a, while the waveform A2b is the amplitude waveform of light transmitted through the other light transmitting portion 100b. When these two amplitude waveforms A2a and A2b are combined, a light intensity distribution as shown in FIG. 46(d) at A3 results, which takes a waveform whose central portion is raised. More specifically, the light intensity distribution loses its sharpness, thus resulting in a blurry image due to the diffraction of the light. This makes it difficult to achieve sharp exposure.
In contrast, when the phase shifter 2 is provided under the one light transmitting portion 100a as illustrated in FIG. 47(a), the blurriness of the image due to the diffraction of light is canceled by inversion of the phase. Thus, a sharp image is transferred and resolution is improved. More specifically, if the phase shifter 2 which provide the one light transmitting portion 100a with a phase shift of 180.degree., for example, light passed through the phase shifter 2 is inverted as shown at B1 (see FIG. 47(b)). Meanwhile, light transmitted through the light transmitting portion 100b does not pass through the phase shifter 2 and therefore the above-described inversion does not take place. Thus, the lights emitted upon a material to be exposed cancel one another at the position shown in FIG. 47(c) at B2. As a result, the light intensity distribution given to the material to be exposed takes an ideal sharp shape as shown in FIG. 47(d) at B3. Thus, the method of forming phase shifters 100a for every other pattern of lines and spaces is generally called Levinson type phase shifter process.
When a defect of a phase shift mask used in the phase shifter process as described above is corrected, if a conventional general method of correcting mask defects is utilized, the following problems are encountered.
FIGS. 48 and 49 are cross sectional views for use in illustration of a conventional method of correcting a bump defect in a phase shift mask. Referring to FIGS. 48 and 49, in the phase shift mask, a phase shifter 2 having a prescribed pattern is formed on a mask substrate 1 formed of silica. A metal light shielding film 3 is formed in a prescribed region on the phase shifter 2. A shifter bump defect 4 is produced when such a phase shift mask is formed.
When such a shifter bump defect 4 is formed in a part of the light transmitting portion originally exclusive of the phase shifter 2, the phase of light corresponding to the portion of the light transmitting portion in which the shifter bump defect 4 is formed will be inverted by 180.degree.. If the inversion of the phase of light takes place in only part of the light transmitting portion, the part of a material to be exposed (resist) corresponding to the light transmitting portion is partially not exposed. If a development is performed in this state, resist will be formed in the region which should originally be without the resist. If a patterning is performed utilizing such resist, a pattern as designed will not be obtained.
Use of an Nd:YAG laser beam 9 for correcting the shifter bump defect 4 can not remove away the shifter bump defect 4, because the Nd:YAG laser beam 9 is transmitted through the shifter defect 4 formed of SOG or SiO.sub.2.
FIGS. 50-52 are cross sectional views of structures for use in illustration of another conventional method of correcting a bump defect in a phase shift mask. According to this method, as can be seen from change in the state from FIG. 50 to FIG. 51, an FIB 5 utilizing Ga ions is irradiated upon the region of the shifter bump defect 4 while scanning the region. Thus, the shifter bump defect 4 is etched away by the FIB 5. Such a technique of etching utilizing an FIB 5 is disclosed in, for example, U.S. Pat. No. 4,548,883. However, since the thickness of the shifter bump defect 4 (about 4000 .ANG.) is about four times as large as the thickness of the black defect 42 (about 1000 .ANG.) in the metal light shielding film illustrated in FIG. 36, the form of the shifter bump defect 4 is rather three-dimensional as compared to the black bump defect 42 in the metal light shielding film. Since the phase shifter 2 should invert the phase of lights by 180.degree., it requires a certain degree of thickness (about 4000 .ANG.). Therefore, the thickness of the shifter bump defect 4 formed at the time of forming the phase shifter 2 naturally becomes as thick as about 4000 .ANG., and the shifter bump defect 4 takes a three dimensional form. It is difficult to evenly scan the shifter bump defect 4 having such a three-dimensional form with an FIB 5. This makes it difficult to etch the shifter bump defect 4 evenly, and the surface of the mask substrate 1 positioned at both lower end portions of the shifter bump defect 4 is etched. Thus, a divot defect 71 as shown in FIG. 52 is produced this time in a surface of the mask substrate 1, and a Ga stain (Ga residue) 72 by Ga ions sticks to the surface of the divot defect 71. The Ga stain 72 which absorbs about 50% of i rays is transferred as a defect at the time of transfer.
As described above, there are various problems associated with conventional methods of correcting bump defects in a phase shift mask.
FIGS. 53 and 54 are cross sectional views of structures for use in illustration of a conventional method of correcting a shifter divot defect in a phase shift mask. As shown in FIG. 53, in this phase shift mask, a phase shifter 12 is formed in a prescribed region on a mask substrate 11. A metal light shielding film 13 is formed in a prescribed region on a phase shifter 12. The shifter divot defect 10 of the phase shifter 12 would be corrected by the following method. More specifically stated, as shown in FIG. 54, as is the case with the conventional method of correcting a white defect in a metal light shielding film described in conjunction with FIGS. 42 to 44, an FIB 5 of Ga ions is irradiated upon the defect region with supply of a hydrocarbon-based gas (deposition gas) 6, while scanning the region. A carbon film 81 is thus formed to fill the shifter divot defect 10. Such technique is disclosed, for example, in U.S. Pat. No. 4,698,236. The carbon film 81 however does not have light transmittance and is transferred as a defect at the time of transfer.
FIGS. 55 and 56 are cross sectional views for use in illustration of another conventional method of correcting a shifter divot defect in a phase shift mask. According to this method, as can be seen from change in the state from FIG. 55 to FIG. 56, an Ar laser beam 19 is irradiated upon the region of a defect while supplying a gas 92 of Mo (Co).sub.6, W (Co).sub.6 or Cr (Co).sub.6 to the defect region. Thus, a metal film 91 of Mo, W or Cr is formed to fill the shifter divot defect 10. However, since the metal film 91 does not have light transmittance either, the same problem as with the method of correction shown in FIGS. 53 and 54 is encountered. More specifically, not having light transmittance, the metal film 91 can be transferred as a defect at the time of transfer.
More specifically, according to the conventional method of correcting a shifter bump defect 4 in a phase shift mask utilizing a laser beam, it has been difficult to remove away the shifter bump defect 4 itself. Further, according to the method of correcting a shifter bump defect utilizing an FIB, the shifter bump defect can be removed, but the surface of the mask substrate 1 is etched as well, generating a divot defect 71 this time, and a Ga stain 72 of Ga ions is formed on the surface of the divot defect 71. According to the method of correcting the shifter divot defect 10 utilizing the FIB 5 and the deposition gas 6, the carbon film 81 formed as a result does not have light transmittance and is transferred as a defect, and the surface of the carbon film 81 reflects the form of the shifter divot defect 10. According to the method of correcting the shifter divot defect 10 utilizing the Ar laser beam 19 and the deposition gas 92, the resultant metal film 91 does not have light transmittance as is the case with the carbon film 81 and is transferred as a defect.